The long range goal of this program remains to gain an understanding of the specificity of helper T cells for protein antigens. In current terms this translates to an understanding of the T cells recognition of peptides containing the antigenic determinant, associated with class II gene products of the MHC, Ia. Antigenic peptides are produced from native antigens by antigen presenting cells (APC) through a series of steps which at present are poorly understood. These involve the uptake of the antigen into an acidic intracellular compartment, the proteolysis of the antigen releasing peptide fragments and the association of these fragments with Ia for display on the APC surface. The immediate goals of this program are: to define the physiochemical properties of the antigenic fragments initially produced by the APC and of those ultimately associated with Ia; to elucidate the steps involved in binding of peptides to MHC and how this process is regulated; to explore alternative or secondary routes for peptide-Ia association; and to attempt to gain information of the structural properties of the peptide bound to the MHC by two dimensional NMR analysis. These studies will be carried out using a well characterized soluble globular protein antigen cytochrome c for which the T cell antigenic determinant is known in detail and into which appropriate labels can be introduced. The results of these studies should lend, valuable insights into the mechanism(s) underlying the T cell recognition of antigen and be of value in the design and synthesis of T cell antigenic peptides for use as both vaccines and immunoregulatory agents.